1. Field of the Invention
The present invention relates to an oscillator using a quartz-crystal element, and more particularly to a high-frequency crystal oscillator facilitating a reduction in size.
2. Description of the Related Art:
In recent years, an increasing need exists for oscillators which stably output signals at such a high frequency as several hundred megahertz or more in various electronic devices. For example, a certain type of digital communication network requires signals at a frequency of 622.08 MHz. Since such an application needs high frequency stability, crystal oscillators are particularly used. For high frequencies, a crystal oscillator with a lower oscillation frequency is typically used and its output is frequency-multiplied by a multiplication circuit to obtain a desired output frequency. In this case, multiplication circuits may be provided in two or three stages in series to obtain a required frequency. Voltage-controlled circuit configurations capable of changing oscillation frequencies to some degree by applied voltage are employed.
FIG. 1 is a circuit diagram showing an example of a configuration of a conventional high-frequency crystal oscillator. The high-frequency oscillator generally comprises voltage-controlled crystal oscillation circuit 1, multiplication amplifier 2, and broadband amplifier 3.
Crystal oscillation circuit 1 operates quartz-crystal element 4 as an inductive element to form a resonant circuit of quartz-crystal element 4 and series capacitors C1, C2, and has transistor 5 therein. A resonance frequency signal of the resonant circuit fed back to transistor 5, and transistor 5 amplified the resonance frequency signal to oscillate crystal oscillation circuit 1. The circuit is a so-called Colpitts crystal oscillation circuit. The oscillation frequency of the circuit is slightly lower than the resonance frequency of the aforementioned resonant circuit due to a circuit capacitance other than the capacitances of series capacitors C1, C2. The oscillation frequency of crystal oscillation circuit 1 is herein referred to as an original frequency and assumed to be 155.52 MHz.
Transistor 5 has a base connected to one end of quartz-crystal element 4, an emitter connected to the midpoint (connection node) of series capacitors C1, C2 and grounded through resistor R5, and a collector connected to power source Vcc through resistor R4. In the resonant circuit, a voltage variable capacitance element, for example variable capacitance diode (varicap diode) 6 is inserted between the other end of quartz-crystal element 4 and a ground point, thereby providing the voltage-controlled resonant circuit. Control voltage Vc serving as a reverse voltage to variable capacitance diode 6 is input through resistor R1 for high frequency blocking and controls the oscillation frequency. Resistors R2, R3 are gate bias resistors for transistor 5.
Multiplication amplifier 2 is a circuit for frequency-multiplying the signal at the original frequency from crystal oscillation circuit 1. Multiplication amplifier 2 has transistor 8 with an emitter grounded and a collector connected to resonant circuit 7 comprising inductor L and capacitor C. The emitter of transistor 8 is connected to a ground point through emitter resistor R8 and bypass capacitor C6 in parallel. Resonant circuit 7 has a resonance frequency set at a value four times higher than the original frequency (155.52 MHz) to obtain an output frequency of 622.08 MHz. Transistor 8 has a base which receives as input the signal at the original frequency from crystal oscillation circuit 1 or an output signal from a multiplication amplifier in the preceding stage through coupling capacitor C3. Resistors R6, R7 are base bias resistors for transistor 8.
It should be noted that capacitor C of resonant circuit 7 is a trimmer capacitor to facilitate the adjustment of resonant circuit 7. Alternatively, inductor L may be a variable inductor. While FIG. 1 shows multiplication amplifier 2 of only one stage, multiplication amplifiers operating as Class-A amplifiers may be connected in a plurality of stages, for example three stages to gradually amplify a frequency signal while waveform distortion is reduced as well as provide amplification up to an input level to the subsequent stage.
Broadband amplifier 3, which is used as a final stage amplifier, amplifies the signal at the output frequency fed from multiplication amplifier 2 in the preceding stage through coupling capacitor C4 to a predetermined output level value with its waveform maintained, and outputs the amplified signal to an external circuit, not shown, through coupling capacitor C5. As broadband amplifier 3, a linear IC (Integrated Circuit) amplifier with a linear output level characteristic to the input level, is used. The linear IC amplifier is employed as the final stage due to its low power consumption and high amplification factor. In this example, supply voltage Vcc is 3.3 V. FIG. 2 is a graph showing the input/output characteristic of the linear IC. The output level is in proportion to the input level until a certain input level. When the certain input level is exceeded, the output level peaks out, that is, the output level becomes substantially constant regardless of the input level.
The aforementioned crystal oscillation circuit 1, multiplication amplifier 2, and broadband amplifier 3 are accommodated as an integral circuit in a shield container to provide the high-frequency crystal oscillator. Specifically, as shown in FIG. 3, quartz-crystal element 4, transistors 5, 8, various elements 11 including resistors, capacitors and inductors, and the linear IC amplifier which constitute crystal oscillation circuit 1, multiplication amplifier 2, and broadband amplifier 3 are mounted on circuit board 10 formed of, for example, a glass fabric base-epoxy resin laminate. A wiring pattern, not shown, is formed on the surface of circuit board 10. Terminals 9 for external connection are attached to circuit substrate 10. Circuit board 10 as described above is fixed to a metal base and covered with a metal cover to complete the high-frequency crystal oscillator accommodated in the shield container.
In the aforementioned conventional high-frequency crystal oscillator, however, the number of used elements such as transistors and passive elements is increased since the original frequency (for example 155.52 MHz) of the crystal oscillation circuit is caused to reach the predetermined output frequency (622.08 MHz) and output level by multiplication amplifiers 2 in a plurality of stages. In addition, the resonant circuit provided for each multiplication amplifier employs a trimmer capacitor or a variable inductor resulting in a larger size to present a problem of an inevitably large size of the circuit as a whole. Furthermore, adjustments required at a number of portions lead to reduced productivity. As the number of elements is increased, troubles occur more frequency to raise the possibility of impairing reliability.
A possible attempt to reduce the number of the stages of the multiplication amplifier is to operate a quartz-crystal element in a crystal oscillation circuit in an overtone mode. In the operation in the overtone mode, however, the frequency changing range when the load capacitance of the quartz-crystal element is changed is significantly narrowed as compared with a case where the quartz-crystal element oscillates at its fundamental frequency mode, and thus a required varying range of frequencies may not be achieved.
It is an object of the present invention to provide a high-frequency crystal oscillator which has a reduced number of circuit elements to promote reductions in size and weight and accomplishes high reliability.
It is another object of the present invention to provide a high-frequency crystal oscillator capable of obtaining a high oscillation output level without adjustments.
The objects of the present invention are achieved by a high-frequency crystal oscillator comprising an oscillation circuit using a quartz-crystal element, means for increasing level of harmonic component in an output from the oscillation circuit, a filter for selecting a component of a predetermined order of the harmonic component, and an amplifier for amplifying the component selected by the filter.
In the present invention, a SAW (Surface Acoustic Wave) filter is preferable as the filter. A Colpitts oscillation circuit operating at the fundamental frequency of a quartz-crystal element is preferable as the oscillation circuit, and a voltage-controlled oscillation circuit is more preferable.
In the present invention, the fundamental frequency of the quartz-crystal element is 50 MHz to 200 MHz, for example. A specific example of the fundamental frequency is 155.52 MHz. The harmonic of the predetermined order is any of harmonics of orders 2 to 8, for example, and typically, a harmonic of order 4. The output frequency of the high-frequency crystal oscillator according to the present invention is typically in the range of 200 MHz to 2 GHz, preferably in the range of 300 MHz to 1.5 GHz, and more preferably in the range of 500 MHz to 1 GHz.
According the present invention, since a high frequency can be obtained without using a multiplication amplifier, the number of circuit elements is reduced, and it is possible to achieve a reduced size, reduced weight, and enhanced reliability in the high-frequency crystal oscillator. In addition, productivity is improved due to no need of adjustments.